Aircraft and aircraft engine design have always strived for reduced weight and greater efficiency. Other factors affecting aircraft and engine design involve cost and size, including the maintenance of the aircraft and the engines. With increased emphasis in these areas, future aircraft are growing in size, requiring either more thrust from the engines or additional engines. Reduced maintenance costs and initial costs can be achieved by enlarging the engines and increasing the thrust provided by the engines rather than by increasing the number of engines. However, as the engines grow larger, weight reduction becomes paramount as larger engines require larger, and therefore, heavier components.
Thus, the next generation of commercial high thrust gas turbine engines will have large fan diameters. The increased fan diameters will require longer blades. The longer blades will have wider chords for increased efficiency. The chord, which is an axial straight line dimension between a trailing edge and a leading edge of a fan blade, will grow with the increased blade size. The wider chord blades offer the increased efficiency because they have greater stability margins and move the air more efficiently across the blade face due to their longer chords. Increased blade efficiency is important in high bypass turbine engines because a significant amount of the air flow generated by the fan blades bypasses the compressors, combustor and turbines and is used to provide direct thrust.
Thus, engine propulsion thrust is typically increased by increasing the diameter of the fan blade/rotor assembly, which also necessarily increases the weight and stress on the fan blades during operation. Accordingly, larger fan blades require correspondingly high strength materials in order to counteract the various aerodynamic and centrifugal stresses generated during operation and for ensuring a suitable useful life span of the larger fan blades.
A typical fan blade includes an airfoil and an integral single tooth attachment root which permits individual assembly and disassembly of the blades in corresponding single tooth attachment slots in a fan rotor disk. The blade dovetail must therefore have sufficient strength for transferring the significant centrifugal loads from the rotating fan blades into the perimeter of the rotor disk within acceptable stress limits. The size and configuration of the airfoil is determined by the specific aerodynamic requirements of the fan and is limited by the availability of suitable high strength materials capable of withstanding the various stresses or stress concentrations experienced during operation of the fan. One problematic area of a fan blade is the stress concentration experienced at the leading edge of the fan blade radially outside of the single tooth attachment.
Titanium is a common high strength material used in fan blades, but is undesirably expensive. A solid titanium fan blade can be readily manufactured, yet has a correspondingly high weight which adds to the centrifugal loads generated during operation. Hollow titanium fan blades are also known for reducing weight while maintaining strength, but increase the complexity of blade manufacture and associated costs. Thus, a hollow titanium blade has minimum weight with suitable high strength yet is very expensive to manufacture. Another form of a titanium fan blade is the hybrid fan blade which is primarily solid titanium with weight reducing pockets formed therein which may be filled with a lightweight, nonstructural filler material to complete the aerodynamic profile of the blade. The hybrid titanium blade is less expensive to manufacture than hollow titanium blades yet does not provide the greater weight reductions of the hollow titanium blade. Hence design methods and/or alternate materials may be a key to weight reduction, while reducing costs.
Further, there is a need to provide such lightweight materials with sufficient strength for transferring the stress concentrations from the rotating fan blades to the perimeter of the rotor disk within acceptable stress limits. More specifically, high stress concentrations at the leading edge of the fan blade radially outside of the single tooth attachment need to be addressed.